1. Field of the Invention
The present invention relates to an oscillating circuit including a resonant element such as a piezoelectric resonator, for example, connected between the differential input terminals of a differential amplifying circuit, and more particularly to a voltage-controlled oscillating circuit which is capable of controlling an oscillation frequency with an external input voltage, and a quartz crystal oscillator incorporating such a voltage-controlled oscillating circuit.
2. Description of the Related Art
Various electronic devices include oscillating circuits. As electronic devices are being scaled down in size, efforts have widely been made to construct oscillating circuits on integrated circuits (ICs). Since it is possible to easily integrate many circuit components such as transistors, resistors, and capacitors in integrated circuits, low-noise oscillating circuits may be constructed by adopting, as a circuit configuration of oscillating circuits, a differential type configuration which has a high power supply noise suppression capability.
An oscillating circuit of a differential type includes a resonant element for determining an oscillation frequency and a differential amplifying circuit connected to the resonant element. The resonant element may be a mechanical resonator or an LC (inductor-capacitor) resonant circuit in addition to a piezoelectric resonator typified by a quartz crystal resonator, i.e., a crystal element or crystal unit.
JP3-230605A discloses an oscillating circuit of a differential type including a differential amplifying circuit which has a pair of bipolar transistors and a resonant element such as a crystal element which is connected between the bases of the bipolar transistors. FIG. 1 shows the oscillating circuit of the differential type disclosed in JP3-230605A.
As shown in FIG. 1, transistors 31, 32 have respective emitters connected in common to a junction that is connected to ground through current source 37. Transistors 31, 32 have collectors respectively supplied with power supply voltage Vcc through load resistors 35a, 35b. Resonant element 21 has an end connected to the base of transistor 31 and the other end connected to the base of transistor 32. Feedback capacitor 33a is connected between the base of transistor 32 and the collector of transistor 31, and feedback capacitor 34a is connected between the base of transistor 31 and the collector of transistor 32. In FIG. 1, capacitors 33b, 34b that are disposed between respective transistors 31, 32 and ground are parasitic feedback capacitors. Power supply voltage Vcc is applied to bias circuit 36, which supplies base bias voltages to the bases of transistors 31, 32 via bias resistors 38a, 38b, respectively. A pair of differential oscillating output signals Lo, /Lo are obtained from the respective collectors of transistors 32, 31.
Since the oscillating circuit shown in FIG. 1 is capable of providing a large loop gain by the differential amplifying circuit, the oscillating circuit is advantageous in that it has a high oscillation start-up capability with low power consumption, or stated otherwise, it has a large negative resistance.
Even if a crystal element or the like is used as resonant element 21, the user may find it desirable to change the oscillation frequency depending on a control voltage supplied from an external circuit. An oscillating circuit which can control its oscillation frequency depending on a control voltage supplied from an external circuit is called a voltage-controlled oscillating (VCO) circuit. It can easily be understood that a voltage-controlled oscillating circuit having a wide variable frequency range is realized by replacing parasitic feedback capacitors 33b, 34b in the circuit shown in FIG. 1 with voltage-controlled variable capacitors.
FIG. 2 shows an example of a voltage-controlled oscillating circuit constructed base on the oscillating circuit shown in FIG. 1. As shown in FIG. 2, transistors Q1, Q2 have respective emitters connected in common to a junction that is connected to ground through current source I1. Transistors Q1, Q2 have collectors respectively supplied with power supply voltage Vcc through load resistors (i.e., collector resistors) R1, R2. Resonant element 21 has an end connected to the base of transistor Q1 through node X1 and the other end connected to the base of transistor Q2 through node X2. Nodes X1, X2 are connected to respective ends of voltage-controlled variable capacitors VC1, VC2, whose other ends are supplied with control voltage Vcont. Therefore, nodes X1, X2 serve as junctions through which variable capacitors VC1, VC2 are connected to resonant element 21, and the bases of transistors Q1, Q2 are connected respectively to nodes X1, X2. Feedback capacitor C1 is connected between node X1 and the collector of transistor Q2, and feedback capacitor C2 is connected between node X2 and the collector of transistor Q1. Bias resistors R3, R4 are connected to respective nodes X1, X2. Bias voltage Vb is applied through bias resistors R3, R4 to the bases of transistors Q1, Q2. Differential buffer amplifier 22 which amplifies the voltage difference between nodes X1, X2 supplies a pair of differential output signals to output terminals OUTPUT.
The voltage-controlled oscillating circuit shown in FIG. 2 can oscillate at frequencies in a wide variable frequency range depending on control voltage Vcont applied to variable capacitors VC1, VC2. However, the oscillating circuit is problematic in that the loop characteristics change greatly depending on the capacitance values of variable capacitors VC1, VC2. Changes in the loop characteristics of the oscillating circuit will be described below with reference to FIG. 3.
FIG. 3 shows loop characteristics in a frequency band in the vicinity of 75 MHz at the time the capacitance values of variable capacitors VC1, VC2 change in a range from 2 pF to 10 pF in the circuit shown in FIG. 2 where a crystal element having a resonant oscillation frequency of 75 MHz is used as resonant element 21. In FIG. 3, curves G1 to G4 represent gain characteristics, and curves P1 to P4 phase characteristics. Curves G1, P1 represent gain and phase characteristics, respectively, at the time the capacitance values of variable capacitors VC1, VC2 are 2 pF each. Curves G2, P2 represent gain and phase characteristics, respectively, at the time the capacitance values of variable capacitors VC1, VC2 are 3 pF each. Curves G3, P3 represent gain and phase characteristics, respectively, at the time the capacitance values of variable capacitors VC1, VC2 are 6 pF. Curves G4, P4 represent gain and phase characteristics, respectively, at the time the capacitance values of variable capacitors VC1, VC2 are 10 pF. The frequency at which the gain is 0 dB or higher and the phase is 0° is an oscillation frequency, and the gain at the frequency represents a loop gain in an oscillating state. As the characteristic curves change more steeply with respect to frequencies, the Q factor of the circuit is greater.
It can be said from FIG. 3 that when the capacitance values of variable capacitors VC1, VC2 change, the loop gain and the Q factor of the circuit vary. Specifically, as the variable capacitances are smaller, i.e., as the oscillation frequency is higher, the loop gain is higher, and the Q factor of the circuit is lower at the same time. A comparison between characteristic curves G1, G4 indicates that characteristic curve G1 is less steep than characteristic curves G4 with respect to frequency changes, and characteristic curve P1 is less steep than characteristic curves P4 with respect to frequency changes, which means a reduction in the Q factor with smaller capacitance values.
Changes in the loop gain and the Q factor affect the characteristics of the voltage-controlled oscillating circuit. Specifically, changes in the loop gain cause the negative resistance to vary, which means that the start-up characteristics of the oscillating circuit are different depending on the control voltage Vcont applied to the voltage-controlled variable capacitors. Reductions in the Q factor of the circuit invite deteriorations of the phase noise.
In particular, it is general practice for voltage-controlled oscillating circuits of the type described to have a differential amplifying circuit and variable capacitors included in an integrated circuit (IC) chip and also to have a resonant element such as a crystal element connected as an external component to the IC chip. It is up to the user to decide on what type of resonant element and what resonant frequency are to be used depending on the purpose of the oscillating circuit. However, it is preferable to use IC chips of one type in combination with resonant elements of different types and different resonant frequencies in view of cost and inventory management. When resonant elements of different types and different resonant frequencies are connected to IC chips of one type, however, the problems of varying start-up characteristics and deteriorating Q factor may possibly be aggravated depending on the resonant elements connected.
It is possible to construct a voltage-controlled oscillating circuit as a package component to be mounted on a wiring board or a circuit board. For example, quartz crystal oscillators which have a crystal element as a resonant element and an IC chip including a differential amplifying circuit and a variable capacitor, all housed in one casing, are widely used in portable devices such as mobile phones because such crystal oscillators can easily be scaled down. The IC chip may incorporate a temperature compensation circuit for compensating for changes in the temperature vs. frequency characteristics of the crystal element.
JP2001-156545A discloses an oscillating circuit including a differential amplifying circuit which has a pair of bipolar transistors and an LC resonant circuit connected between the collectors of the transistors.